Synthesis, Characterization and Photophysical Properties of Naphtho[2,3-b]Indolizine-6,11-Dione Derivatives

Article abstract of DOI:10.1007/s10895-015-1687-x

This communication concerns the synthesis, structural characterization and fluorescent properties of novel naphtho[2,3-b]indolizine-6,11-dione derivatives. The target compound synthesized by one-pot multicomponent reaction between naphthoquinone, indandio

Full text of DOI:10.1007/s10895-015-1687-x

J Fluoresc
DOI 10.1007/s10895-015-1687-x
SHORT COMMUNICATION
Synthesis, Characterization and Photophysical Properties
of Naphtho[2,3-b]Indolizine-6,11-Dione Derivatives
Nelli Batenko1 & Ilze Neibolte1 & Sergey Belyakov1,2 & Raimonds Valters1
Received: 7 June 2015 /Accepted: 7 October 2015
#
Springer Science+Business Media New York 2015
Abstract This communication concerns the synthesis, struc-
tural characterization and fluorescent properties of novel
naphtho[2,3-b]indolizine-6,11-dione derivatives. The target
compound synthesized by one-pot multicomponent reaction
between naphthoquinone, indandione and pyridine deriva-
tives. The structures of synthesized compounds were charac-
terized by IR, 1H NMR and 13C NMR analysis. The structure
of two compounds was confirmed by single crystal X-ray
diffraction. The photophysical properties of synthesized com-
pounds were measured in a variety of organic solvents. The
effects of substituents and solvents on UV-vis spectra and
fluorescence emission were analyzed. All compounds were
showed negative solvatochromism. The fluorescence emis-
sion of these derivatives was sensitive to solvent basicity
and the large Stokes shifts (up to 129 nm, 4599 cm−1) were
observed. The similar multicomponent reaction between
naphthoquinone, bindone and pyridine derivatives provide
the formation of stable non-fluorescent compound possessing
zwitter-ionic structure.
Introduction
The chemistry of natural and non-natural quinones has attracted
the attention of chemists due to a vast number of applications
[1, 2]. Quinone derivatives exhibit ground and excited state
reactivities and are highly suitable organocatalysts [3].
Quinone based receptors can be used to signal the qualitative
or quantitative presence of ions. Quinones are highly coloured
compounds and good fluorophores showing excitation in the
range 380–500 nm with the wide emission bands [4].
Fluorescent molecular probes that emit in the visible region
and whose fluorescence emission is sensitive to environment
and solvent polarity are of significant interest due to their ver-
satile applications in chemistry, biology and environmental sci-
ence [5]. Developing new fluorophore scaffolds remains a very
challenging task. To date, the oft-used dyes in bio-chemical
research are still limited to a few chemical families, such as
rhodamine, BODIPY and coumarin derivatives. Small
fluorophore molecules are often preferred, as they are generally
easier to synthesize and their smaller volumes allow them to
access tiny cavities, i.e., to reach certain protein targets or semi-
conductor interfaces, thus becoming more useful [6]. The intro-
duction of different chemical substituents into the molecular
frameworks can improve important properties of fluorophore
such as large Stokes shift, large molar extinction coefficient,
and high quantum efficiency, but only some general guide-
lines are considered for the design of new dyes at the pres-
ent time [6].
.
Keywords Naphtho[2,3-b]indolizine-6,11-dione Fused
.
.
.
heterocycles Stokes shift Negative solvatochromism
Solvent basicity
* Nelli Batenko
The present study was aimed to design and synthesize a
series of naphtho[2,3-b]indolizinedione derivatives bearing
electron-donating and electron-withdrawing substituents and
evaluated their photophysical properties and solvatochromism
behavior. All the synthesized derivatives exhibited interesting
photophysical properties such as large Stokes shift, consider-
able fluorescent quantum yield and sensitivity to solvent
nelli.batenko@rtu.lv
1
Faculty of Materials Science and Applied Chemistry, Riga Technical
University, 3/7 P. Valdena St., Riga LV-1048, Latvia
2
Latvian Institute of Organic Chemistry, 21 Aizkraukles St.,
Riga LV-1006, Latvia

J Fluoresc
basicity. The target molecules showed bathochromic shift by
substituting the electron-donating groups at C-2 position and
hypsochromic shift by increasing conjugation compared to
unsubstituted product. The synthetic protocol for the synthesis
of naphtho[2,3-b]indolizinedione derivatives was quite simple
(one-pot multi-component reaction) and give the target com-
pounds without involvement of complex chromatographic
techniques for purification of compounds.
(50 ml). Solution was refluxed for 1 h and then 1 eq. of H2O2
(30 % aqueous solution) was added to a stirring solution. After
15 min N-heterocycle was added (8.8 mmol) and reaction
mixture was refluxed for 2 h and then 1 eq. of H2O2 (30 %
aqueous solution) was added. The rate of the reaction was
followed by TLC. Reaction mixture was refluxed for 2–6 h
and 1 eq. of H2O2 (30 % aqueous solution) can be added if
TLC (acetone/hexane 1:1) data shows the presence of the blue
intermediate. Reaction mixture was cooled to room tempera-
ture and kept at this temperature for 24 h. The orange (red)
precipitate that formed was filtered and washed with acetoni-
trile. Pure product can be obtained by recrystallization.
Experimental
Instrumentations
Reagents and solvents were purified by standard means or
used without further purification.
2-(6,11-Dioxo-6,11-Dihydronaphtho[2,3-b]
Indolizine-12-Carbonyl)Benzoic Acid (4a)
The IR spectra were recorded in pellets with KBr on a
Thermo Electron Nicolet 5700 instrument. H-NMR and
1
Rf = 0.28, yield 68 %, mp 259 °C. IR, ν, cm−1: 1623, 1661,
1719, 2667, 3064. UV (CHCl3), λmax, nm (lg ε): 327 (4.08),
493 (3.73). 1H-NMR (DMSO-d6), δ, ppm (J, Hz): 7.51 (1 H,
td, J = 1.3, 7.0), 7.58–7.60 (3 H, m), 7.68–7.80 (4 H, m), 7.85
(1 H, dd, J = 1.6, 7.1), 8.15 (1 H, dd, J = 0.8, 7.6), 8.28 (1 H, d,
J = 9.0), 9.82 (1 H, d, J = 6.0) 12.80 (1 H, br. s). 13C-NMR
(DMSO-d6), δ, ppm: 113.5; 119.2; 121.1; 121.7; 126.2; 126.9;
127.7; 128.4; 128.8; 129.3; 130.3; 130.8; 131.5; 132.3; 133.4;
133.8; 133.9; 134.7; 140.1; 142.8; 169.0 (C = O); 174.7
(C = O); 180.4 (C = O); 191.7 (C = O). Found, %: C 72.62;
H 3.19; N 3.37. C24H13NO5. Calcd., %: C 72.91; H 3.31; N
3.54. Crystal data: monoclinic; a = 7.0822(2), b = 16.5816(4),
c = 18.0600(6) Å, β = 93.588(1)°; V = 2116.7(1) Å3, Z = 4,
μ = 0.347 mm−1, Dcalc = 1.507 g cm−3; space group is P21/c.1.
13C-NMR spectra were recorded on a Bruker Avance 300
spectrometer at 300 and 75 MHz respectively, with DMSO-
d6 and CDCl3 as solvent and HMDS (δ, 0.05 ppm) as internal
standard. J values are given in Hz. Melting points were mea-
sured on Kruess KSP 11 Melting Point Analyzer. Diffraction
data were collected at −80 °C (4a) and −90 °C (6b) on a
Bruker-Nonius KappaCCD diffractometer using graphite
monochromated Mo-Kα radiation (λ = 0.71073 Å). The crys-
tal structure was solved by direct methods [7, 8] and refined
by full-matrix least squares [9]. The UV-Vis absorption spec-
tra with concentration ~10−5 M at room temperature (~298 K)
were acquired using Perkin-Elmer 35 UV/Vis spectrometer
with 1 cm path length quartz cuvettes.
Emission spectra in CHCl3 or DMF solutions with concen-
tration ~10−5 M were measured at room temperature (~298 K)
on QuantaMaster 40 steady state spectrofluorometer (Photon
Technology International, Inc.) using 1 cm path length quartz
cuvettes.
Emission lifetime measurements were performed on
PicoMaster 1 TCSPC fluorescence lifetime spectrofluorome-
ter (Photon Technology International, Inc.) using 410 nm,
445 nm or 460 nm nanosecond pulsed light emitting diodes
(Photon Technology International, Inc.) as excitation source at
room temperature. The TCSPC signal was detected using a
photomultiplier tube (PMC-100-20, Becker & Hickl).
Absolute emission quantum yields in DMF solutions with
concentration ~ 10−4M were determined using QuantaMaster
40 steady state spectrofluorometer (Photon Technology
International, Inc.) equipped with 6 in. integrating sphere by
LabSphere.
2-(2,3-Dimethyl-6,11-Dioxo-6,11-Dihydronaphtho[2,3-b]
Indolizine-12-Carbonyl)Benzoic Acid (4b)
Rf = 0.08, yield 62 %, mp 273 °C. IR, ν, cm−1: 1622, 1670,
1719, 2919, 3208. UV (DMF), λmax, nm (lg ε): 336 (4.28),
491 (3.93). 1H-NMR (DMSO-d6), δ, ppm (J, Hz): 2.37 (3 H,
s), 2.42 (3 H, s), 7.54 (1 H, dd, J = 2.5; 6.3), 7.57–7.62 (2 H,
m), 7.70–7.72 (2 H, m), 7.77 (1 H, dd, J = 1.7; 4.9), 7.81 (1 H,
dd, J = 1.7; 7.0), 8.08 (2 H, d, J = 10.1), 9.55 (1 H, s), 12.67–
12.76 (1 H, br. s). 13C-NMR (DMSO-d6), δ, ppm: 17.3; 20.2;
112.5; 119.5; 120.9; 125.6; 126.1; 126.9; 127.5; 128.6; 129.2;
129.9; 130.4; 133.6; 131.5; 132.2; 133.4; 133.9; 134.5; 139.6;
142.7; 143.0; 169.0 (C = O); 174.2 (C = O); 180.6 (C = O);
191.8 (C = O). Found, %: C 73.48; H 3.83; N 3.09.
C26H17NO5. Calcd., %: C 73.75; H 4.05; N 3.31.
Synthesis of Compounds 4a-g
1 For further details, see crystallographic data for 4a deposited at the
Cambridge Crystallographic Data Centre as Supplementary
Publications with number CCDC 1029264.
2,3-Dichloro-1,4-naphthoquinone (1.0 g, 4.4 mmol) and 1,3-
indandione (0.64 g, 4.4 mmol) were dissolved in acetonitrile

J Fluoresc
2-(2-Methyl-6,11-Dioxo-6,11-Dihydronaphtho[2,3-b]
Indolizine-12-Carbonyl)Benzoic Acid (4c)
479 (3.51). 1H-NMR (DMSO-d6), δ, ppm (J, Hz): 7.58–7.62
(3 H, m), 7.75 (2 H, d, J = 6.4), 7.82–7.86 (2 H, m), 8.00 (1 H,
dd, J = 1.6, 7.3), 8.14 (1 H, d, J = 7.3), 8.34 (2 H, d, J = 6.2),
8.73 (1 H, s), 8.98 (2 H, d, J = 6.2), 9.84 (1 H, d, J = 7.3),
12.82–12.86 (1 H, br. s). 13C-NMR (DMSO-d6), δ: 115.4;
117.2; 120.1; 122.4; 123.8; 126.4; 127.1; 128.2; 128.5;
128.8; 128.9; 129.4; 130.9; 131.2; 131.5; 132.4; 133.5;
133.8; 134.2; 134.7; 135.5; 139.3; 142.3; 145.2; 150.2;
169.0 (C = O); 175.2 (C = O); 180.2 (C = O); 191.8 (C = O)
ppm. Found, %: C 73.59; H 3.23; N 5.77. C29H16N2O5.
Calcd., %: C 73.73; H 3.41; N 5.93.
Rf = 0.82, yield 61 %, mp 264 °C. IR, ν, cm−1: 1618, 1672,
1716, 2638, 3080. UV (CHCl3), λmax, nm (lg ε): 335 (4.19),
504 (3.79). 1H-NMR (DMSO-d6), δ, ppm (J, Hz): 2.50 (3 H,
s), 7.37 (1 H, dd, J = 1.7, 7.1), 7.55–7.64 (3 H, m), 7.68–7.74
(2 H, m), 7.79 (1 H, dd, J = 1.5, 4.7), 7.83 (1 H, dd, J = 1.7,
7.1), 8.11 (2 H, d, J = 7.6), 9.68 (1 H, d, J = 7.1), 12.65–12.80
(1 H, br. s). 13C-NMR (DMSO-d6), δ, ppm: 21.7 (CH3);
112.5; 119.5; 121.5; 121.6; 126.1; 126.9; 127.8; 128.0;
128.6; 129.5; 130.5; 131.5; 132.2; 133.5; 133.7; 133.8;
134.5; 140.6; 141.8; 142.9; 169.0 (C = O); 174.4 (C = O);
180.5(C = O); 191.7 (C = O). Found, %: C 72.99; H 3.66; N
3.36. C25H15NO5. Calcd., %: C 73.35; H 3.69; N 3.42.
12-(2-Carboxybenzoyl)
-6,11-Dioxo-6,11-Dihydronaphtho[2,3-b]
Indolizine-2-Carboxylic Acid (4 g)
2-(2-t-Butyl-6,11-Dioxo-6,11-Dihydronaphtho[2,3-b]
Indolizine-12-Carbonyl)Benzoic Acid (4d)
Rf = 0.18, yield 63 %, mp 307 °C. IR, ν, cm−1: 1627, 1673,
1689, 2658, 2821, 3103. UV (DMF), λmax, nm (lg ε): 337
1
(3.56), 478 (3.21). H-NMR (DMSO-d6), δ, ppm (J, Hz):
Rf = 0.30, yield 59 %, mp 252 °C. IR, ν, cm−1: 1620, 1674,
1698, 2967, 3410. UV (CHCl3), λmax, nm (lg ε): 335 (4.29),
507 (3.85). 1H-NMR (DMSO-d6), δ, ppm (J, Hz): 1.36 (9 H,
s), 7.54–7.56 (3 H, m), 7.66 (1 H, dd, J = 2.0, 7.4), 7.78 (4 H,
qd, J = 7.7, 13.7), 8.14 (2 H, m), 9.69 (1 H, d, J = 7.3), 12.75–
12.86 (1 H, br. s). 13C-NMR (DMSO-d6), δ: 30.3 (CH3); 35.6;
112.9; 115.3; 118.5; 121.6; 126.2; 126.9; 127.9; 128.2; 128.5;
129.4; 130.5; 131.5; 132.5; 133.6; 133.8; 133.9; 134.5; 140.4;
143.1; 153.8; 168.9 (C = O); 174.6 (C = O); 180.5 (C = O);
191.74 (C = O) ppm. Found, %: C 74.38; H 4.43; N 2.92.
C28H21NO5. Calcd., %: C 74.49; H 4.69; N 3.10.
7.62 (3 H, dd, J = 2.0, 7.4), 7.75 (2 H, dd, J = 3.3, 6.1), 7.81
(2 H, m), 7.88 (1 H, d, J = 6.9), 8.17 (2 H, d, J = 7.9), 9.82
(1 H, dd, J = 1.1, 7.3), 12.82–12.90 (1 H, br. s), 13.60–13.80
(1 H, br. s). 13C-NMR (DMSO-d6), δ: 120.7; 122.42; 127.63;
128.01; 131.2; 131.9; 133.2; 133.9; 135.5; 135.7; 136.4;
138.2; 138.6; 139.0; 139.5; 143.5; 143.6; 145.8; 147.1;
155.7; 170.5 (C = O); 173.8 (C = O); 180.0 (C = O); 185.0
(C = O); 196.7 (C = O) ppm. Found, %: C 68.17; H 2.76; N
3.01. C25H13NO7. Calcd., %: C 68.34; H 2.98; N 3.19.
Method for the preparation of compounds 6a,b: 2,3-
Dichloro-1,4-naphthoquinone (1.0 g, 4.4 mmol) and bindone
(1.21 g, 4.4 mmol) were dissolved in acetonitrile (50 ml).
Solution was refluxed for 1 h and then 1 eq. of H2O2 (30 %
aqueous solution) was added to a stirring solution. After
15 min pyridine 3a or 3b was added (8.8 mmol) and reaction
mixture was refluxed for 2–4 h. The rate of the reaction was
followed by TLC. Reaction mixture was cooled to room tem-
perature and kept at this temperature for 24 h. The black pre-
cipitate that formed was filtered. Pure product can be obtained
by recrystallization from ethanol.
Inner salt 6a: Rf = 0.18, yield 62 %, mp > 300 °C. λmax
(CHCl3) nm (lg ε): 503 (4.04), 535 (3.98), 742 (3.79). λmax
(DMF) nm: 498, 527, 704. 1H-NMR (CDCl3), δ, ppm (J, Hz):
7.23 (1 H, m), 7.43 (1 H, m), 7.49 (2 H, m), 7.60 (1 H, m), 7.69
(1 H, dd, J = 1.5, 7.4), 7.73 (2 H, dd, J = 1.7, 3.2), 7.77 (1 H, dd,
J = 1.5, 7.4), 7.80 (1 H, m), 8.02 (2 H, dt, J = 3.9, 7.6), 8.19 (1 H,
dd, J = 1.3, 7.6), 8.32 (1 H, m), 8.83 (1 H, d, J = 7.7), 9.07 (1 H,
d, J = 6.1), 9.48 (1 H, d, J = 6.0). 13C-NMR (CDCl3), δ: 120.5;
121.3; 125.0; 126.0; 126.1; 126.6; 127.7; 128.7; 130.1; 130.8;
131.5; 132.5; 132.6; 133.0 (2C); 133.1; 133.8; 134.2; 135.1;
140.9; 142.1; 143.0; 144.6; 145.1; 146.7; 148.0; 150.1; 160.1;
162.3; 177.8 (C = O); 180.2 (C = O); 197.5 (C = O); 218.4
(C = O) ppm. Found, %: C 77.97; H 3.17; N 2.59.
C33H17NO5. Calcd., %: C 78.10; H 3.38; N 2.76.
2-(8,13-Dioxo-8,13-Dihydro [5, 6]Indolo[2,1-a]
Isoquinoline-14-Carbonyl)Benzoic Acid (4e)
Rf = 0.63, yield 67 %, mp 293 °C. IR, ν, cm−1: 1640, 1667,
1744, 2652, 3064. UV (CHCl3), λmax, nm (lg ε): 306 (4.33),
470 (3.56). 1H-NMR (DMSO-d6), δ, ppm (J, Hz): 7.48 (1 H,
d, J = 8.6), 7.54 (1 H, d, J = 7.6) 7.64 (2 H, d, J = 4.4), 7.68
(2 H, d, J = 7.0), 7.80 (2 H, d, J = 7.8), 7.95 (3 H, m), 8.19
(2 H, dd, J = 4.2, 7.7), 9.45 (1 H, d, J = 7.4), 13.15–13.25 (1 H,
br. s). 13C-NMR (DMSO-d6), δ: 116.6; 118.3; 122.8; 124.1;
124.5; 125.2; 126.7; 126.8; 127.6; 128.3; 128.4; 129.3; 129.8;
130.0; 130.4; 132.0; 133.1; 133.2; 133.8; 134.1; 134.5; 134.7;
135.8; 135.9; 170.9 (C = O); 175.0 (C = O); 180.7 (C = O);
193.3 (C = O). Found, %: C 75.33; H 3.33; N 3.05.
C28H15NO5. Calcd., %: C 75.50; H 3.39; N 3.14.
2-(6,11-Dioxo-2-(Pyridin-4-yl)
-6,11-Dihydronaphtho[2,3-b]Indolizine-12-Carbonyl)
Benzoic Acid (4f)
Rf = 0.40, yield 68 %, mp 320 °C. IR, ν, cm−1: 1630, 1669,
1700, 2626, 3066. UV (DMF), λmax, nm (lg ε): 340 (3.81),

J Fluoresc
Inner salt 6b: Rf = 0.36, yield 58 %, mp > 300 °C. λmax
1
(CHCl3) nm (lg ε): 503 (3.94), 533 (3.72), 733 (3.67). H-
NMR (CDCl3), δ, ppm (J, Hz): 2.53 (3 H, s), 2.58 (3 H, s),
7.26 (1 H, m), 7.42 (2 H, m), 7.48–7.52 (2 H, m), 7.60 (1 H,
m), 7.70 (2 H, m), 7.76 (1 H, dd, J = 1.5, 7.4), 8.01 (2 H, m), 8.17
(1 H, d, J = 7.3), 8.76 (1 H, d, J = 7.6), 9.00 (1 H, s), 9.10 (1 H, d,
J = 6.5). Found, %: C 78.39; H 3.76; N 2.45. C35H21NO5.
Calcd., %: C 78.50; H 3.95; N 2.62. Crystal data: monoclinic;
a = 12.4943(4), b = 13.8701(4), c = 16.7828(5) Å,
β = 105.505(2)°; V = 2802.6(2) Å3, Z = 4, μ = 0.094 mm−1,
D
calc = 1.378 g cm−3; space group is P21/n.2.
Results and Discussions
Synthesis
Scheme 1 Synthesis of substituted naphtho[2,3-b]indolizinediones 4a-
g.Reagents and conditions: H2O2, acetonitrile, reflux, 3–10 h
are observed in the crystal structure of compound 6b is prob-
ably the reason for the stability of zwitterions.
Previously, Ayyangar et al. [10] reported that reaction of
naphthoquinone 1 with 1,3-indandione (2) and pyridine (3a) in
dioxane provide the formation of the mixture of 2-(6,11-dioxo-6,
11-dihydronaphtho[2,3-b]indolizine-12-carbonyl)benzoic acid’s
derivatives. These compounds contain indolizine core – a com-
mon substructure in a number of biologically important natural
products and pharmaceuticals and is used to provide a high level
of fluorescence [11]. Previously described procedure [10] affords
4a only in 20 % yield. This fact prompted us to modify the
conditions of the reaction. The modified procedure (Scheme 1)
includes reaction between quinone 1 and 1,3-indandione (2) in
boiling acetonitrile with following addition of H2O2. On the next
stage 2 equivalents of N-heterocycle 3a-g must be added to the
reaction mixture. After refluxing and subsequent oxidation by
hydrogen peroxide the final product 4a-g can be easily isolated
in moderate yield (59–68 %). Interestingly, that the reaction of
naphthoquinone 1, pyridine derivatives 3a,b and 1,3-indandione
analogue – bindone (5) (Scheme 2) in the similar reaction con-
ditions provide the formation of non-fluorescent zwitter-ionic
compound 6a,b. The new zwitterions are air and moisture-stable.
The first step of a possible mechanism of the formation of
compounds 4a-g and 6a,b involves nucleophilic attack of ac-
tivated methylene group of 1,3-indandione or bindone at C2-
position of naphthoquinone and chlorine atom substitution.
Nucleophilic attack by pyridine nitrogen and chlorine atom
substitution at C3-position of naphthoquinone proceeds on
the next stage. The final polycyclic structure of compounds
4a-g was formed after loss of HCl molecule, followed by
cyclization step, and indandione cycle oxidative cleavage.
The zwitter-ionic structure of 6a,b was formed after loss of
HCl molecule. The fact that intramolecular hydrogen bonds
X-ray Study
The structure of 4a (Fig. 1) and 6b (Fig.2) was confirmed by
single crystal X-ray diffraction. In accordance with X-ray diffrac-
tion data the compound 4a gives monoclinic centrosymmetric
molecular crystals. The asymmetric unit contains two molecules,
one of them is a molecule of the compound and the second is
solvent (CH2Cl2) molecule. In molecules of the naphthoquinone
derivative the tetracyclic fragment is characterized by planar con-
formation. The values of torsion angles, which characterize the
conformation of 2-carboxybenzoyl substituent are following:
C11a − C12 − C7’ − C1’ = 41.6(9), C12 − C7’ −X C1’
− C6’ = 24.5(9)°. The bond lengths and valence angles in whole
correspond to classical structure formula 4a. In the crystal struc-
ture by means of strong intermolecular hydrogen bonds of
2 For further details, see crystallographic data deposited at the Cambridge
Crystallographic Data Centre as Supplementary Publications with num-
ber CCDC 1035590
Scheme 2 Synthesis of zwitter-ionic compounds 6a,b. Reagents and
conditions: H2O2, acetonitrile, reflux, 2–4 h

J Fluoresc
chloroform
DMF
acetone
EtOH
400
450
500
550
600
Wavelength (nm)
Fig. 3 UV-visible absorption spectra of 4d in varieties of solvents
In accordance with X-ray analysis the compound 6b crystal-
lizes as an ethanol solvate. The ethanol molecules are bonded
with molecules 6b by means of hydrogen bonds of OH•••O type.
These bonds are relatively strong; the H-bond length (O5•••O6
distance) is equal 2.767(2) Å (H•••O5 = 1.88(4) Å,
O6 − H•••O5 = 177(3)°). The analysis of the bond lengths and
valence angles shows that in the molecular structure there are
neither pure double bonds nor pure single bonds, but rather a
hybrid of mesomeric forms. In the molecular structure there is an
intramolecular hydrogen bond of CH•••O type. The carbon
atoms in the second and sixth positions of the pyridinium ring
are characterized by heightened electronegativity due to bonding
with N+. This promotes the formation of CH•••O hydrogen
bonds. The parameters of the found hydrogen bond are follow-
ing: C6•••O4 = 3.280(3) Å, H•••O4 = 2.33(3) Å,
C6 − H•••O4 = 164(2)°. Although this bond is not strong, it
impedes a free rotation of pyridinium cycle. On the other hand,
pure theoretically the intramolecular hydrogen bond
C2 − H•••O4 is also presumable. Thus, beyond 6b, its rotamer
(atropisomer) with the C2 − H•••O4 bond should be stable.
Probably, these interactions stabilize zwitter-ionic structure and
prevent subsequent transformations.
Fig. 1 The structure of 4a, showing the atom-numbering scheme and
displacement ellipsoids are drawn at the 50 % probability level (H atoms
are shown as small spheres of arbitrary radii)
OH•••O type (O•••O = 2.610(7) Å, H•••O = 1.66 Å,
O − H•••O = 169°) the molecules form centrosymmetric dimers,
which are usual for carbonic acids. Intermolecular contact
O7’•••C1s (3.459(8) Å) between molecules and solvent can be
considered as weak hydrogen bond of CH•••O type
(H•••O = 2.61 Å, O − H•••O = 148°).
Absorption and Fluorescence Emission Studies
The UV-visible properties, fluorescence emission characteristics
and photophysical properties of the target compounds 4a-g are
summarized in Figs. 3, 4 and 5 and Table 1. To evaluate the effect
of solvent polarities on the absorption and emission
chloroform
DMF
acetone
EtOH
Fig. 2 The structure of molecular complex 6b with ethanol, showing the
atom displacement ellipsoids (at 50 % probability level). For the sake of
clarity, labels of carbon atoms have been omitted, except for atoms
involved in possible hydrogen bonds
500
600
Wavelength (nm)
Fig. 4 Fluorescence emission spectra of 4d in varieties of solvents
700
800

J Fluoresc
lowest energy absorption bands of derivatives 4b-g consistently
blue shifted by 13 nm (4e, R = benzo[a]) - 30 nm (4 g,
R = COOH). The maximum red shift (509 nm) was observed
in chloroform in case of 4b. Moreover, compounds 4b-d bearing
electron-donating substituents showed red shift in all solvents, at
the same time compound 4e with extended π-system demonstrat-
ed blue shift (11–20 nm) in all solvents compared to 4a.
Interestingly, that no significant substituent effect on the absorp-
tion band of 4f,g in DMF, although the lowest energy bands of
these compounds are blue shifted in other solvents compared to
4a. Blue shift in acetone can be described by solvent’s different
ability to form hydrogen bonds. All compounds 4a-g were
showed negative solvatochromism. These observations led to
the conclusion that ground state is relatively polar and better
stabilized by solvation than the excited state [12]. In compounds
4a-g the electron-donating substituent had red shift effect on the
lowest energy absorption band and electron withdrawing substit-
uent provide blue shift effect on the lowest energy absorption
band.
4000
3500
3000
2500
R² = 0.9422
R² = 0.9803
0
0.2
0.4
0.6
0.8
Catalan SB parameter
Fig. 5 Correlation of solvent induced Stokes’ shift of 4a,d with Catalán
SB parameter
characteristics of the naphtho[2,3-b]indolizinediones 4a-g, all the
synthesized compounds were tested in the different solvents in-
cluding chloroform, acetone, dimethylformamide and ethanol. In
some cases (Table 1) due to limited solubility UV/vis and fluo-
rescence spectra were recorded for saturated solutions only.
In the UV-vis spectra of compound 4a (with no substituent in
the pyridine ring, R = H) the lowest energy absorption band is
blue shifted by 24 nm in acetone compared to chloroform. The
Fluorescence emission spectra of 4a-d,g in chloroform
were showed emission maxima at 568–583 nm and only
Table 1 Observed UV-visible
absorption maxima, molar
absorptivity, fluorescence
emission maxima, Stokes shift
[Δλ (nm) and Δυ (cm−1)],
fluorescence lifetime and absolute
quantum yield of compounds 4a-
g in different solvents
Compound
Solvent
λ
abs, nm
ε (M−1cm−1
)
λ
em, nm
Δλ, nm
Δυ, cm−1
τ
FL, ns
ΦFL
4a
CHCl3
acetone
DMF
493
469
476
476
509
483
491
490
504
479
487
489
507
478
484
489
470
457
462
465
479
469
488
458
478
460
5400
5209
7733
-*
572
563
578
592
577
577
590
604
568
577
590
606
583
578
592
611
557
553
563
578
579
598
573
551
589
581
79
2800
3560
3710
4120
2320
3370
3420
3850
2240
3550
3580
3950
2570
3620
3770
4080
3320
3800
3880
4200
3600
4600
3040
3690
3940
4530
–
–
94
–
–
102
116
68
7.7
–
0.22
–
EtOH
CHCl3
acetone
DMF
4b
4c
4d
4e
-*
–
–
-*
94
–
–
8599
-*
99
3.2
–
0.20
–
EtOH
CHCl3
acetone
DMF
114
64
6127
6958
6950
-*
–
–
98
–
–
103
117
76
11.4
–
0.18
–
EtOH
CHCl3
acetone
DMF
7048
6376
8920
-*
–
–
100
108
122
87
–
–
10.5
–
0.21
–
EtOH
CHCl3
acetone
DMF
3660
5344
7600
-*
–
–
96
–
–
101
113
100
129
85
7.2
–
0.36
–
EtOH
DMF
4f
3196
-*
6.2
–
0.25
–
EtOH
CHCl3
acetone
DMF
4 g
-*
–
–
-*
93
–
–
1652
2196
111
121
5.8
–
0.17
–
EtOH
*Saturated solution

J Fluoresc
compound 4e was demonstrated blue shift (557 nm).
Compounds 4b-d bearing electron-donating substituents showed
red shift in various solvents (acetone, DMF and EtOH) compared
to 4a. The maximum red shift (611 nm) was observed in ethanol
in case of 4d. The maximum emission wavelength and Stokes
shift for each compound are summarized in Table 1. The maxi-
mum Stokes shift for compounds 4a-g was observed in DMF
(99–111 nm) and ethanol (113–122 nm). Derivatives 4a-g con-
tain carbonyl and carboxyl groups and stabilization due to hy-
drogen bonding interaction with solvent is expected in excited
state. Different solvent parameters (Z scale, π* scale, solvent
dipole moment, Reichardt-Dimroroth’s ET(30) scale [12], solvent
polarizability scale [13], Catalán solvent basicity [14], hydrogen-
bond basicity of solvent [15]) were examined to establish the
correlation between Stokes shift and solvent properties. In solu-
tions specific solvent-fluorophore interaction is possible and the
extent of this interaction can depend on the functional groups
present in the fluorophore. Correlation using Catalán solvent
basicity (SB) scale provides a more appropriate estimate of the
solvent induced Stokes shift. In the case of 4d (Fig. 5) all the data
points of the correlation between Stokes shift and Catalán SB
parameter lie on the straight line with a correlation coefficient of
0.98. In cases of other compounds the correlation coefficients lie
in range 0.89–0.98. For example, correlation for compounds 4a
(R = 0.94) and 4b-d (R > 0.95) bearing electron donating sub-
stituent was good or excellent. Interestingly, that correlation be-
tween Stokes shift and similar β1 parameter (hydrogen-bond
basicity of solvent [15]) is worse and give correlation coefficients
in range 0.74–0.92.
solvent basicity parameter. Naphtho[2,3-b]indolizinedione deriv-
atives bearing electron donating substituents are potential candi-
dates for use as solvent basicity probes.
Acknowledgments Support for this work was provided by the Riga
Technical University through the Scientific Research Project Competition
for Young Researchers No 2013/10.
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Conclusion
Multi-component reaction between 2,3-dichloronaphthoquinone,
N-heterocycle and compound containing activated methylene
group (1,3-indandione or bindone) provide the formation of
two different products: fluorescent naphtho[2,3-b-
]indolizinedione derivatives 4a-g or non-fluorescent zwitter-ionic
compounds 6a,b. Several naphtho[2,3-b]indolizinedione deriva-
tives bearing electron donating and electron withdrawal substit-
uents were synthesized. Their absorption and fluorescence emis-
sion were studied in several solvents. All compounds were
showed negative solvatochromism. The absorption maximum
of these derivatives depends upon the functional groups in the
fluorophore. The fluorescence emission maxima showed
bathochromic shift, considerable fluorescent quantum yield and
large Stokes shift. Stokes shifts were correlated with Catalán
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